System for photonic computing

ABSTRACT

A system for photonic computing, preferably including an input module, computation module, and/or control module, wherein the computation module preferably includes one or more filter banks and/or detectors. A photonic filter bank system, preferably including two waveguides and a plurality of optical filters optically coupled to one or more of the waveguides. A method for photonic computing, preferably including controlling a computation module, controlling an input module, and/or receiving outputs from the computation module.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Nonprovisional application Ser.No. 16/672,231 filed 1 Nov. 2019 which claims the benefit of U.S.Provisional Application Ser. No. 62/757,647, filed on 8 Nov. 2018, bothof which are incorporated in its entirety by this reference.

TECHNICAL FIELD

This invention relates generally to the photonic computing field, andmore specifically to a new and useful system and method for photoniccomputing.

BACKGROUND

Typical photonic filter bank systems may suffer from high noise,detuning due to temperature changes, and/or other effects that reducefilter bank performance. Thus, there is a need in the photonic computingfield to create a new and useful system and method for photoniccomputing.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A-1B are schematic representations of an embodiment of the systemand an example of the embodiment, respectively.

FIGS. 2A-2C are schematic representations of a first, second, and thirdembodiment, respectively, of the input module.

FIGS. 3A-3B are schematic representations of an embodiment of thecomputation module and an example of the embodiment, respectively.

FIGS. 4A-4C are schematic representations of a first, second, and thirdvariation, respectively, of a spectral filter bank.

FIGS. 5A-5B are schematic representations of a first and secondembodiment, respectively, of a phase weight bank.

FIGS. 5C-5D are schematic representations of a first and secondembodiment, respectively, of an amplitude weight bank.

FIGS. 6A-6B are schematic representations of a first alternateembodiment of a portion of the computation module and an example of thefirst alternate embodiment, respectively.

FIGS. 7A-7B are schematic representations of a first and second example,respectively, of photonic elements of the system.

FIGS. 8A-8B are schematic representations of a first and second example,respectively, of elements of an alternate embodiment of the computationmodule.

FIG. 8C is a schematic representation of an example of an alternateembodiments of the computation module.

FIG. 9 is a schematic representation of an embodiment of the method.

FIGS. 10A-10B are schematic representations of optical channels andfilter resonances of a first and second example, respectively, of thesystem.

FIGS. 11A-11B are schematic representations of a first and secondexample, respectively, of a phase weight bank with nested modulatorelements.

FIGS. 12A-12D are schematic representations of various examples of adouble-modulator assisted phase weight bank.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments of the inventionis not intended to limit the invention to these preferred embodiments,but rather to enable any person skilled in the art to make and use thisinvention.

1. System

A system 100 for photonic computing preferably includes an input module110, computation module 120, and/or control module 130 (e.g., as shownin FIGS. 1A-1B). In some embodiments, the system includes one or moreelements such as described in U.S. Pat. No. 10,009,135, issued 26 Jun.2018 and titled “System and Method for Photonic Processing”, which ishereby incorporated in its entirety by this reference. However, thesystem can additionally or alternatively include any other suitableelements.

The system and/or elements thereof are preferably implemented as one ormore integrated circuits. For example, the photonic modules (e.g., inputmodule, computation module) and/or subsets thereof can be and/or includeone or more photonic integrated circuits, and/or the entire system canbe a portion of a single integrated circuit. However, the system canadditionally or alternatively be implemented in any other suitabledevice structure(s).

1.1 Input Module

The input module 110 preferably functions to generate a photonicrepresentation of an input signal. The input signal is preferablyrepresentative of an input vector (e.g., encodes the input vector). Theinput module preferably includes one or more transducers and amultiplexer, and can additionally or alternatively include any othersuitable elements.

1.1.1 Transducers

The transducers preferably function to control light emission at variouswavelengths. The input module preferably includes a plurality oftransducers. Each transducer preferably controls a different emissionchannel (e.g., wavelength channel, mode channel, etc.). For example,each transducer can control a different emission channel near (e.g.,within a threshold distance of, substantially centered around, etc.) the1.3 micron and/or 1.55 micron wavelength (e.g., within the 1.26-1.36micron O-band, within the 1.53-1.565 micron C-band and/or the1.565-1.625 micron L-band, etc.), wherein the wavelengths describedherein preferably refer to the wavelength the light would have in freespace, rather than to the wavelength of the light in the medium throughwhich it is propagating. The wavelength channels are preferablynarrow-band channels, such as channels of less than a thresholdbandwidth (e.g., 1, 2, 5, 10, 15, 25, 40, 65, 100, 200, 500, 1000 GHz,1-5, 5-20, 20-100, 100-300, and/or 300-1000 GHz frequency bandwidth;0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, 0.01-0.05, 0.05-0.2,0.2-1, 1-3, or 3-10 nm spectral bandwidth; etc.), but can additionallyor alternatively include intermediate- and/or wide-band channels and/orchannels of any other suitable widths. The channels can additionally oralternatively be associated with optical modes (e.g., transverse spatialmodes, polarization modes, etc.) and/or any other suitable opticalcharacteristics. Alternatively, a single transducer can control multipleemission channels, and/or the transducers can emit light of any othersuitable wavelength(s) and/or other optical characteristics. Eachchannel preferably corresponds to a different element of the inputvector.

The channels are preferably non-overlapping, more preferably having atleast (and/or at most) a threshold spacing (e.g., threshold amountrelative to the channel width, such as 5, 10, 25, 50, 100, 110, 125,150, 175, 200, 250, 300, 400, 500, 0-1, 1-5, 5-15, 15-30, 30-60, 60-100,100-110, 110-120, 120-150, 150-200, 200-300, or 300-500% of the channelwidth; absolute threshold amount, such as 0.1, 0.2, 0.5, 1, 2, 5, 10,15, 20, 30, 40, 50, 75, 100, 150, 250, 0.1-1, 1-5, 5-15, 15-45, 45-100,or 100-300 GHz; etc.) between each other (e.g., center-to-centerdistance, edge-to-edge distance, etc.). However, all or some of thechannels can alternatively be overlapping (e.g., by no more and/or noless than a threshold amount, such as described above regarding thethreshold spacing) and/or have any other suitable relationship to eachother. The channels and/or emitters can be indexed based on wavelength(e.g., from shortest to longest wavelength, such as channel 1 beingassociated with the shortest wavelength, channel 2 being associated withthe second shortest wavelength, etc.).

The transducer preferably couples light into one or more structures(e.g., on a chip), such as waveguides. The transducer is preferably anoptical transducer, more preferably an electro-optical transducer (e.g.,which outputs lights based on an electrical input), but can additionallyor alternatively be any other suitable transducer. For example, theinput module can include one or more transducers such as described inU.S. Pat. No. 10,009,135, issued 26 Jun. 2018 and titled “System andMethod for Photonic Processing”, which is hereby incorporated in itsentirety by this reference.

In one embodiment, each transducer includes an emitter and an amplitudemodulator. The input module can additionally or alternatively includemultiple transducers that receive light from a shared emitter, such aswherein the shared emitter emits light (e.g., unmodulated orsubstantially unmodulated light) corresponding to multiple opticalchannels (e.g., multiple wavelength channels), such as shown by way ofexample in FIG. 2D. The emitter is preferably a laser (e.g., diodelaser, preferably a component of an integrated circuit), such as adistributed feedback (DFB) laser, a distributed Bragg reflector (DBR)laser, a Fabry Perot (FP) cavity laser (e.g., with multiple modes,thereby outputting light of multiple wavelengths) such as a quantum dot-and/or quantum well-based FP cavity laser, an external cavity laser(e.g., optionally including one or more integrated modulators), amode-locked laser (e.g., gain-absorber system) configured to outputlight of multiple wavelengths, an optical frequency comb (OFC), and/or avertical cavity surface emitting laser, but can additionally oralternatively include an LED and/or any other suitable light emitter. Insome examples, the emitter (e.g., DFB laser emitting a singlewavelength, DBR laser emitting multiple wavelengths, etc.) can becoupled to (output light to) one or more modulators (e.g., Mach-Zehndermodulators), wherein the modulators are driven by one or more varying(e.g., oscillating) signals, thereby generating additional opticalchannels. In some examples, the emitter (e.g., DFB laser emitting asingle wavelength, DBR laser emitting multiple wavelengths, etc.) can becoupled to (output light to) one or more nonlinear optical elements(e.g., silicon nitride ring exhibiting nonlinear optical effects). Thetransducer preferably includes one amplitude modulator for each emitterand/or each channel. The amplitude modulator is preferably an opticalmodulator, but can additionally or alternatively be an emitter modulatoror any other suitable modulator.

The optical modulator preferably functions to modulate light emitted byan emitter (or multiple emitters). The optical modulator is preferablywavelength-selective (e.g., substantially modulating only a narrowwavelength band, such as substantially modulating only light of a singlechannel), but can alternatively be a wideband modulator and/or have anyother suitable wavelength dependence. The optical modulator can beelectro-absorptive and/or electro-refractive. The optical modulator canoptionally be embedded in one or more other structures, such as aresonator and/or Mach-Zehnder interferometer (MZI), which can functionto enhance its modulation performance. In examples, the opticalmodulator can include one or more microresonators (e.g., microringresonator, microdisk resonator, photonic crystal defect statemodulator), quantum confined Stark effect (QCSE) modulator, Zeno effectmodulator (e.g., graphene based modulator, such as a silicon photonicgraphene modulator), MZI modulator, electro-absorptive modulatorembedded in a critically coupled resonator (e.g., QCSE microdiskmodulator), photonic crystal-based modulator, and/or any other suitableoptical modulator. The optical modulator (e.g., wideband modulator) canoptionally be embedded in and/or in series (along the optical path) withone or more filters (e.g., spectral filters), such as anelectro-absorptive modulator preceded (along the optical path) by afirst filter and followed by a second filter. In some variations, theoptical modulator includes multiple microresonators (e.g., as describedin U.S. patent application Ser. No. 16/374,991, filed 4 Apr. 2019 andtitled “Photonic Filter Bank System and Method of Use”, which is herebyincorporated in its entirety by this reference; as shown in FIGS. 5Band/or 5D; etc.). The optical modulators can additionally oralternatively include mode modulators (e.g., as described in Lian-WeeLuo, Noam Ophir, Christine P. Chen, Lucas H. Gabrielli, Carl B. Poitras,Keren Bergmen, and Michal Lipson, “WDM-compatible mode-divisionmultiplexing on a silicon chip,” Nat. commun. 5, 3069 (2014), which ishereby incorporated in its entirety by this reference). In somevariations, the optical modulator includes multiple filters and/ormodulators coupled together using inverse design (e.g., as described inWeiliang Jin, Sean Molesky, Zin Lin, Kai-Mei C. Fu, and Alejandro W.Rodriguez, “Inverse design of compact multimode cavity couplers,” Opt.Express 26, 26713-26721 (2018), which is hereby incorporated in itsentirety by this reference). However, the system can additionally oralternatively include any other suitable optical modulators, or includeno such modulators.

The emitter modulator can function to control light emission from theemitter (or from multiple emitters). For example, the emitter modulatorcan provide an electrical signal that drives the associated emitter, orthere can be no emitter modulator, wherein the input signal (e.g.,electrical signal, such as from the control module) directly drives theemitter. In a specific example, in which the transducer is a laserdevice, the modulated laser gain medium can be an active opticalsemiconductor, which can act as a subthreshold temporal integrator withtime-constant equal to carrier recombination lifetime. The laser deviceitself can act as a threshold detector, rapidly dumping energy stored inthe gain medium into the optical mode when the net gain of the cavitycrosses unity (e.g., similar to a passively Q-switched laser biasedbelow threshold). However, the input modulator (e.g., amplitudemodulator) can additionally or alternatively include any other suitableemitter modulator(s), and/or any other suitable modulators of anykind(s).

The transducers can additionally or alternatively include any othersuitable elements. The transducers of the input module can besubstantially the same as each other (e.g., aside from emitting atand/or modulating different wavelengths), or can be different from oneanother.

1.1.2 Multiplexer

The multiplexer preferably functions to combine multiple optical signals(e.g., channels) onto a single output path (e.g., a waveguide), such asfor wavelength-division multiplexing (WDM). The multiplexer ispreferably an optical multiplexer, such as an arrayed waveguide grating(AWG), but can additionally or alternatively be any other suitablemultiplexer.

In a first embodiment of the input module, signals (e.g., unmodulatedsignals) from multiple emitters are combined by a multiplexer, thenmodulators (preferably wavelength-selective modulators, such asmicroresonators) alter the multiplexed signals (e.g., as shown in FIG.2A). Preferably, each wavelength-selective modulator alters a singlesignal, wherein the other signals (e.g., wavelengths) pass throughand/or by the modulator substantially unaltered. Additionally oralternatively, some or all of the modulators can substantially affectmore than one of the signals.

In a second embodiment, modulated signals from multiple transducers(e.g., optical transducers, preferably electro-optical transducers) arecombined by a multiplexer. In a first example of this embodiment, anoptical modulator is arranged between each emitter and the multiplexer(e.g., as shown in FIG. 2B). In a second example, an emitter modulatorcontrols each emitter (e.g., as shown in FIG. 2C).

The input module is preferably controlled by the control module (e.g.,by electrical signals from the control module). The input modulepreferably outputs to the computation module (e.g., the WDM opticalsignal is sent to an input of the computation module, preferably along awaveguide). However, the input module can additionally or alternativelyinterface with other elements of the system in any other suitablemanner, and/or the input module can additionally or alternativelyinclude any other suitable elements in any suitable arrangement.

1.2 Computation Module

The computation module 120 preferably functions to perform photoniccomputations (e.g., matrix multiplication) based on signals from theinput and/or control modules. For example, the computation module canmultiply an input vector (e.g., encoded by the WDM signal received fromthe input module) by a matrix (e.g., associated with the input signalsfrom the control module) to determine an output vector (e.g., associatedwith output signals generated by the computation module). Thecomputation module preferably includes one or more spectral filter banks122 and detectors 123, and can optionally include one or more splitters121 (e.g., as shown in FIGS. 3A, 3B, 7A, and/or 7B). However, thecomputation module can additionally or alternatively include any othersuitable elements.

1.2.1 Spectral Filter Banks

Each spectral filter 122 bank preferably functions to filter (e.g.,filter in a substantially time-independent manner; switch, such as at alow rate; modulate at a high rate, such as comparable to the bandwidthof the input optical signal; otherwise control; etc.) an optical signalbased on a control signal, wherein such filtering can include alteringthe amplitude (e.g., diminishing and/or amplifying), phase (e.g.,delaying and/or reducing delay), and/or any other suitable opticalcharacteristics. Each spectral filter bank preferably includes a set offilter elements, more preferably wherein each filter element isassociated with (e.g., filters) a channel (or set of channels) of theinput signal. The computation module preferably includes a plurality ofspectral filter banks (e.g., each corresponding to a row of the matrix).Each spectral filter bank preferably receives (e.g., at an IN port ofthe spectral filter bank) an optical signal input (e.g., WDM signal) anda set of control signals. The optical signal input is preferablyreceived from the splitter (e.g., along one or more of the paths ontowhich the signal is split), but can additionally or alternatively bereceived from any other suitable element. The control signals (e.g.,filter weights) are preferably received from the control module, but canadditionally or alternatively be received from any other suitableelement. The control signals are preferably electrical signals (e.g.,voltage signals). The control signals preferably control operation ofone or more filter elements of the spectral filter bank. The controlsignals preferably include one weight for each filter element, but canadditionally or alternatively include any other suitable number ofweights. In some embodiments, the number of filters and number ofweights can be equal to the number of channels in the optical signalinput (e.g., equal to the number of emitters in the input module). Eachweight and filter can correspond to an element of the matrix rowassociated with the spectral filter bank (or to an integer number ofsuch elements, such as 2, 4, 8, 16, 32, 2-8, 9-32, etc.). The spectralfilter bank preferably outputs the filtered optical signal(s) (e.g., toone or more detectors). In some examples, the spectral filter bank hasmultiple optical outputs (e.g., THRU port and DROP port, OUT1 port andOUT2 port, etc.), one or more of which outputs to a detector (e.g., asshown in FIGS. 4A-4C). The spectral filter bank(s) can optionallyinclude weight banks (and/or elements thereof) such as described in U.S.Pat. No. 10,009,135, issued 26 Jun. 2018 and titled “System and Methodfor Photonic Processing”, which is hereby incorporated in its entiretyby this reference (e.g., as described regarding the MRR weight bank,such as employing microring resonators, microdisk resonators, photoniccrystal-based resonators, any modulators described above regarding theinput modulators, and/or any other suitable filters, etc.), such asshown by way of example in FIG. 5C. A person of skill in the art willrecognize that each filter element can include a single filteringdevice, multiple filtering devices (e.g., arranged in series and/orparallel), multiple tunable elements, and/or any other suitable elementscapable of filtering a channel (or set of channels) of the input signal.

The filter elements (e.g., of the spectral filter banks) preferablyfunction to filter the optical input signal. In a first embodiment ofthe spectral filter bank, every channel propagates past all the filterelements of the spectral filter bank (e.g., as shown in FIGS. 5A-5D). Inthis embodiment, the filter elements are preferably wavelength-selectiveoptical filters (e.g., substantially filtering only a narrow wavelengthband, such as substantially filtering only light of a single channel).The wavelength-selective optical filters are preferably microresonators(e.g., more preferably microdisc resonators, but additionally oralternatively including microring resonators, photonic crystal defectstate filters, etc.).

In some embodiments, each microresonator includes a plurality of p-njunctions (preferably, a large number of p-n junctions, such as at least8, 16, 32, 64, 128, 256, 512, or 1024 junctions, etc.), which can enablethe microresonator to accept one or more digital control signals (e.g.,rather than analog control signals). In a first example, themicroresonator accepts a unary control signal (e.g., thermometer code),such as described by way of example in Moazeni, Sajjad, Sen Lin, MarkWade, Luca Alloatti, Rajeev J. Ram, Miloš Popović, and VladimirStojanović, “A 40-Gb/s PAM-4 transmitter based on a ring-resonatoroptical DAC in 45-nm SOI CMOS”, IEEE Journal of Solid-State Circuits 52,no. 12 (2017): 3503-3516, which is herein incorporated in its entiretyby this reference. In a second example, the microresonator accepts abinary control signal, preferably wherein each bit of the binary controlsignal is configured to control a different number of p-n junctions,depending on the bit's significance (e.g., wherein the least significantbit drives 1 junction, the next bit drives 2 junctions, the next bitdrives 4 junctions, the next bit drives 8 junctions, and so on,preferably with the most significant bit driving approximately half ofthe total number of junctions). However, the microresonator canadditionally or alternatively accept any other suitable control signalsand/or other electrical inputs. The microresonators (e.g., microdiscs,microrings, etc.) can have any suitable doping geometry (e.g., with theplurality of p-n junctions in any suitable arrangement), such as, forexample, interleaved, interior ridge, and/or zig-zag doping geometries.

The optical filter can optionally be embedded in one or more otherstructures, such as a resonator and/or Mach-Zehnder interferometer(MZI), which can function to enhance its modulation performance and/oralter the modulation mechanism. In some variations, the optical filterincludes multiple microresonators (e.g., as described in Alexander N.Tait, Allie X. Wu, Thomas Ferreira de Lima, Mitchell A. Nahmias, BhavinJ. Shastri, and Paul R. Prucnal, “Two-pole microring weight banks,” Opt.Lett. 43, 2276-2279 (2018), which is hereby incorporated in its entiretyby this reference; as shown by way of example in FIG. 5B). In somevariations, the optical filter includes multiple filters and/ormodulators coupled together using inverse design (e.g., as described inWeiliang Jin, Sean Molesky, Zin Lin, Kai-Mei C. Fu, and Alejandro W.Rodriguez, “Inverse design of compact multimode cavity couplers,” Opt.Express 26, 26713-26721 (2018), which is hereby incorporated in itsentirety by this reference). In some variations, the optical filterincludes one or more nested modulator elements, such as modulatorelements including one or more microresonators (e.g., microrings,microdiscs, etc.) coupled to one or more outer feedback arms (e.g.,coupled to the microresonator in an add/drop configuration, such asshown by way of example in FIGS. 11A-11B and/or 12C-12D). For example,the optical filter can include one or more nested modulator elementssuch as described in “S. Darmawan, Y. M. Landobada, and M. K. Chin,“Nested ring Mach-Zehnder interferometer”, Opt. Express 15, 437-448(2007), which is hereby incorporated in its entirety by this reference.

Each filter of a spectral filter bank (e.g., weight bank) preferably hasa different resonance wavelength (e.g., resonance wavelength under fixedconditions, such as a typical operating temperature and no appliedvoltage). Preferably, each resonance wavelength corresponds to (e.g., iswithin, such as substantially centered within) a different wavelengthchannel (e.g., as shown in FIGS. 10A-10B).

In a second embodiment, each channel is split onto a different sub-pathto interact with a filter element associated with that channel. In afirst example of this embodiment, the modulated path includes for eachchannel: a drop filter to branch a sub-path off the main path, a filteron that sub-path, and an add filter to rejoin the signal from thesub-path to the main path. In this example, the filter is preferably amicroresonator (e.g., microring resonator, microdisk resonator, etc.),but can additionally or alternatively include a Bragg filter (e.g.,fiber Bragg grating; Bragg reflector, preferably with a mirror andcirculator such as a monolithic Bragg reflector with an optical loopmirror and a circulator; etc.) and/or any other suitable filter. In asecond example, the modulated path includes a demultiplexer to create aplurality of sub-paths, a filter on each sub-path, and a multiplexer torecombine the sub-paths following modulation. In this embodiment, eachfilter can be an electro-refractive element, a microresonator, and/orany other suitable filter. However, the spectral filter bank canadditionally or alternatively have any other suitable arrangement and/orcan define any other suitable optical path(s).

Preferably, each spectral filter bank 122 is a phase weight bank 122 a.The phase weight bank preferably includes two (or more) paths, aplurality of phase modulator elements, and a coupler (e.g., as shown inFIG. 5A). However, the phase weight bank can additionally oralternatively include any other suitable elements. Each path (e.g.,waveguide) preferably receives an input from an output path of thesplitter, more preferably receiving identical WDM input signals on eachpath. In some such embodiments, some or all of the phase weight banksreceive the inputs from a respective two-way splitting element of thesplitter (e.g., wherein the two-way splitting element receives an inputfrom earlier stage(s) of the splitter, and splits the element into twooutputs, one for each path of the phase weight bank). For example, oneor more of the phase weight banks can be preceded by a directionalcoupler (e.g., wherein the directional coupler and phase weight bankcooperatively define an MZI), such as wherein a first portion of theinput to the directional coupler, preferably including substantiallyhalf of the optical power, is delivered as the first output; and asecond portion of the input to the directional coupler, preferably alsoincluding substantially half of the optical power, is delivered as thesecond output (e.g., wherein the second output is substantiallyidentical to the first output, but possibly phase-shifted relative tothe first output, such as by π/2).

Each phase modulator element preferably modulates the phase of a channel(e.g., a wavelength channel) on a path (or both paths) of the phaseweight bank, wherein a person of ordinary skill in the art willrecognize that modulating the phase of a channel can include one or moreof: phase-shifting light of the channel in a substantiallytime-independent manner; switching phase-shifting of light of thechannel, such as at a low rate; modulating phase-shifting of light ofthe channel at a high rate, such as comparable to the bandwidth of theinput optical signal; and/or controlling the phase of light of thechannel in any other suitable manner. Preferably, all the modulatorelements modulate channels on the same path, but different channels canalternatively be modulated on different paths from each other. Themodulator elements are preferably wavelength-specific modulators (e.g.,microresonators), but can additionally or alternatively include anyother suitable optical phase modulators. In some examples, one or moreof the channels can be modulated by a pair of modulator elements (e.g.,microresonators such as microrings and/or microdiscs, etc.), the twomodulator elements of the pair modulate light on different paths,preferably cooperatively operating as a double-modulator (e.g.,double-ring) assisted MZI (e.g., as shown by way of examples in FIGS.12A-12D). The two modulator elements of the pair can have (and/or betuned to) different resonances, such as resonance wavelengths on eitherside of the wavelength channel (target wavelength) they are configuredto modulate (e.g., as described in Liangjun Lu, Linjie Zhou, Xinwan Li,and Jianping Chen, “Low-power 2×2 silicon electro-optic switches basedon double-ring assisted Mach-Zehnder interferometers”, Opt. Lett. 39,1633-1636 (2014), which is hereby incorporated in its entirety by thisreference), preferably wherein the resonance wavelengths are closer(e.g., much closer, such as by a factor of at least 1.5, 2, 3, 5, 10,20, 50, 1.5-3, 3-10, 10-30, and/or 30-100, etc.; slightly closer, suchas by a factor between 1 and 1.5; etc.) to the target wavelength than towavelengths of any other channel modulated by the filter bank. However,the filter bank can additionally or alternatively include any othersuitable modulator elements.

The coupler preferably functions to couple the two paths of the phaseweight bank past the phase modulator elements (e.g., to couple lightpropagating on the two paths after it has been phase-modulated by thephase modulator elements of the phase weight bank). For example, thecoupler can be a directional coupler, such as a coupler includingcoupled waveguide segments. The interference between the signals on thetwo paths preferably generates an amplitude signal based on the signals'phase differences (e.g., phase differences imposed by the phasemodulators). In some examples (e.g., in which a substantially equalsignal intensity is provided at each input of the phase weight bank,such as corresponding to an intensity value of 1), if the signals on thetwo waveguides reach the coupler with substantially no phase difference(e.g., if no phase shift is imposed by the phase filter, the inputsignals are in phase, and the two waveguides have substantially equaloptical path lengths; if a phase shift imposed by the phase filtersubstantially cancels a phase difference arising from offsets in theinput phases and/or differences in the optical path lengths; etc.), theoutput intensity at each arm can be substantially equal to each other(e.g., resulting in a 0 output value at a balanced photodetector pair),such as having an intensity value of approximately 1 (e.g., ignoringtransmission losses); by varying the phase shift, the output intensitiescan be varied, preferably from a first extremum (e.g., at a π/2 phaseshift) in which substantially all output intensity appears on thewaveguide with the phase filter(s) and substantially none appears on theother waveguide, to a second extremum (e.g., at a −π/2 phase shift) inwhich substantially no output intensity appears on the waveguide withthe phase filter(s) and substantially all appears on the otherwaveguide.

The computation module can additionally or alternatively include one ormore amplitude weight banks and/or any other suitable spectral filterbanks. The amplitude weight banks can include weight banks (and/orelements thereof) such as described in U.S. Pat. No. 10,009,135, issued26 Jun. 2018 and titled “System and Method for Photonic Processing”,which is hereby incorporated in its entirety by this reference (e.g., asdescribed regarding the MRR weight bank; similar to the MRR weight bankbut with modulators other than microrings, such as any modulatorsdescribed above regarding the input module amplitude modulators; etc.),such as shown by way of example in FIG. 5C.

However, the system can additionally or alternatively include any othersuitable optical filters. Although referred to herein as opticalfilters, a person of skill in the art will recognize that the filterscan additionally or alternatively include optical switches, opticalmodulators, and/or any other suitable elements.

1.2.2 Detectors

Each detector 123 preferably functions to transduce an optical signal(e.g., into an electrical signal). The computation module preferablyincludes one detector (e.g., summation detector) associated with eachspectral filter bank. However, the computation module can alternativelycombine signals from multiple spectral filter banks, wherein thecombined signal is input to a single detector. The detectors preferablyinclude one or more photodetectors (e.g., photodiodes), but canadditionally or alternatively include any other suitable detectors. In afirst embodiment, each detector includes a pair of photodiodes (e.g.,balanced photodetector), such as one each on the THRU and DROP ports ofthe spectral filter bank (e.g., as shown in FIG. 5C). In a secondexample, the detector is a single photodiode (e.g., on either the THRUor the DROP port). However, the detector can additionally oralternatively include any other suitable arrangement of photodiodesand/or other detectors. Each detector output and/or derivatives thereof,such as combinations of detector outputs (e.g., sums or differences ofmultiple detector outputs) is preferably delivered to the control module(e.g., as an electrical signal). However, one or more detector outputscan additionally or alternatively be used to drive one or moretransducers (e.g., transducers of the same input module, of anotherinput module, etc.). For example, the detector outputs can be used todrive transducers such as described in U.S. Pat. No. 10,009,135, issued26 Jun. 2018 and titled “System and Method for Photonic Processing”,which is hereby incorporated in its entirety by this reference (e.g., asdescribed regarding FIG. 2 of U.S. Pat. No. 10,009,135).

1.2.3 Splitter

The splitter preferably functions to split a signal (e.g., received fromthe input module), propagating the split signal along a plurality ofpaths (e.g., waveguides). The number of paths onto which the signal issplit is preferably based on the number of spectral filter banks in thecomputation module (e.g., one path for each weight bank, two paths foreach weight bank, three paths for each weight bank, etc.). For example,the signal can be split onto two paths for each phase weight bank andone path for each amplitude weight bank.

The splitting is preferably wavelength-independent; alternatively,different wavelength selective elements can be used to split eachchannel (or set of multiple channels, such as adjacent channels)independently. The signal is preferably split equally (or substantiallyequally) between all paths and/or spectral filter banks, but canalternatively be split with any other suitable intensity distribution.The splitter can include one or more splitter elements, such as two-waysplitters, star couplers, multi-mode interference (MMI) couplers,inverse design couplers, and/or any other suitable elements. In oneexample, the splitter is a tree splitter, including a plurality ofsplitter elements in a tree configuration (e.g., including a pluralityof two-way splitters arranged in a binary tree). In some variations, thetree splitter can include elements of one or more of the above types.For example, a plurality of 1×k couplers (i.e., couplers that split asingle input into k paths) can be combined in serial layers to provide Noutputs.

In a first embodiment, the signal from the input module is splitdirectly and propagated to all of the spectral filter banks (e.g., asshown in FIG. 3B).

In a second embodiment, splitters are interspersed with (and/orintegrated with) spectral filter banks. In this embodiment, the splitterelements and filter banks can be arranged in a tree structure (e.g.,binary tree structure, such as shown in FIG. 6A; tree of star couplers,such as described above; etc.). In a specific example, the spectralfilter banks are integrated with a set of MZIs (e.g., a spectral filterbank on one path of each MZI), wherein the THRU port of any given MZI isfed to the IN port of a first downstream MZI and the DROP port of thegiven MZI is fed to the IN port of a second downstream MZI (e.g., asshown in FIG. 6B).

However, the system can additionally or alternatively include any othersuitable splitter(s) in any suitable arrangement, or can include nosplitter (e.g., wherein the computation module includes a singlespectral filter bank and detector which filter the optical inputsignal).

1.2.4 Alternate Embodiments

In alternate embodiments, the computation module is configured toperform matrix operations (e.g., matrix multiplication, such asvector-matrix multiplication) for matrices of sizes greater than thenumber of optical channels (e.g., the number of transducers). Forexample, such embodiments can including expanding the input signal to Kmultiple optical streams (e.g., in the form of multiple inputwaveguides), each with L channels (e.g., wavelengths), for a total inputvector of length K·L, while still allowing full generality to animplementation of any real matrix using linear optical components. Forexample, such an embodiment could implement a 64×64 matrix operation byusing 8 waveguides and 8 wavelengths per waveguide. In theseembodiments, the input module 110 can optionally include multiplewaveguides (e.g., K waveguides), preferably including substantiallyidentical WDM signals, and/or the computation module 120 can include asplitter that couples the optical signal from the input module intomultiple waveguides.

In some such embodiments, multiple matrix operations (e.g., usingmatrices of smaller size than the desired matrix, such as matriceslimited in size to the number of wavelengths) can be performed,producing an equivalent result as the desired matrix operation. In suchembodiments, to implement a matrix operation (e.g., matrix-vectormultiplication M·

) using a N×KL matrix M (e.g., wherein N mod K=0), for each of the Loptical characteristic channels (e.g., wavelengths), denoted as λ_(i)for 0≤i<L, the K input streams are preferably mapped to N outputwaveguides, corresponding to an N×K matrix M_(λ) _(i) . Based on thedesired N×K matrix, the method can include determining (e.g., usingsingular value decomposition, preferably a reduced SVD such as the thinSVD) two unitary matrices, U_(λ) _(i) and V_(λ) _(i) , and a diagonalmatrix, Σ_(λ) _(i) , wherein M_(λ) _(i) =U_(λ) _(i) Σ_(λ) _(i) V*_(λ)_(i) where V*_(λ) _(i) is the conjugate transpose of V_(λ) _(i) . Forexample (e.g., using the thin SVD), U_(λ) _(i) , can be an N×K unitarymatrix, Σ_(λ) _(i) can be a K×K diagonal matrix, and V_(λ) _(i) can be aK×K unitary matrix. In one example, a series of optical elements areused to first calculate

=V*·

, then calculate

=Σ·

=ΣV*·

, and finally calculate the desired result,

=U·

=UΣV*·

=M·

, wherein

is determined by outputting each of the N separate multi-channel (e.g.,WDM) paths resulting from multiplication by U to a different detector(e.g., photodetector), each of which sums the L channels of the outputinto a single value, corresponding to an element of

. The unitary matrices can, for example, be implemented optically usingone or more of a variety of decomposition techniques (e.g., as describedin William R. Clements, Peter C. Humphreys, Benjamin J. Metcalf, W.Steven Kolthammer, and Ian A. Walmsley, “Optimal design for universalmultiport interferometers,” Optica 3, 1460-1465 (2016), the entirety ofwhich is incorporated herein by this reference; similar to thedescription of Clements et al., but optically implementing an N×Kunitary matrix, wherein K<N, by implementing an N×N unitary matrix suchas described in Clements et al., and providing the appropriate inputsignals to K of the N input waveguides, while providing no input signalto the remaining N−K input waveguides and/or not including anywaveguides that carry substantially no optical power). The diagonalmatrix is preferably implemented using modulators on each waveguide,which can be of any form (e.g., as described above regarding the system,such as regarding the spectral filter banks and/or any other suitableelements of the computation module). In some variations, one or more ofthe matrices (e.g., the unitary matrices) are implemented using one ormore MZIs 124, preferably MZIs including one or more phase weight banks(e.g., inside and/or outside the interferometer), such as dual-inputMZIs 124 a (e.g., as shown in FIG. 8A) and/or single-input MZIs 124 b(e.g., as shown in FIG. 8B).

In one example (e.g., as shown in FIG. 8C), in which the computationmodule 120 includes 4 wavelengths and 2 input waveguides, an 8×8 matrixis divided into a unitary 8×2 matrix (U), a diagonal 2×2 matrix (Σ), anda unitary 2×2 matrix (V). In this example, the computation moduleincludes a plurality of MZIs, such as wherein each phase weight bankincludes a MZI that includes phase weight banks inside and outside theinterferometer. For each wavelength, this component can implement a 2×2unitary matrix operation (e.g., for a matrix belonging to the SU(2) Liegroup) for each wavelength, preferably independently and simultaneously(or substantially concurrently), but additionally or alternatively, atdifferent times and/or having any suitable interdependence. A person ofskill in the art will recognize that this configuration can be extendedto any other suitable number of wavelengths and/or input waveguides.

However, the computation module can additionally or alternativelyinclude any other suitable elements in any suitable arrangement.

1.3 Control Module

The control module 130 preferably functions to control and/or receiveoutputs from the other elements of the system. The control modulepreferably controls (e.g., provides electrical control signals to) thetransducers of the input module and/or the spectral filter banks (e.g.,the modulators) of the computation module. The control module preferablyreceives outputs (e.g., electrical signals) from the detectors. Thecontrol of the transducers, filters and/or other elements can optionallybe altered based on the received outputs.

The control module can include, for example, one or more one or moreprocessors, preferably electronic processors (e.g., CPU, GPU,microprocessor, FPGA, ASIC, etc.), storage elements (e.g., RAM, flash,magnetic disk drive, etc.), look up tables, serializers, deserializers,digital to analog converters (e.g., which can function to generatecontrol signals for the transducers, filters, and/or other controlledelements), analog to digital converters (e.g., which can function toencode the detector output signals), and/or any other suitable elements.

However, the system can additionally or alternatively include any othersuitable elements in any suitable arrangement.

1.4 Material Platforms

The system can include (e.g., be made of) any suitable materials. Thesystem (and/or elements thereof, such as some or all of the photonicelements) can be implemented on one or more material platforms, such asphotonic integrated circuit platforms (e.g., silicon photonicsplatforms, zero-change photonic platforms, other photonic platforms,etc.), microelectronic platforms, and/or any other suitable materialplatforms. In some examples, the system is fabricated via co-integration(e.g., between electronics and photonics), such as wherein differentelements of the system can be joined together using one or morepackaging technologies such as flip chip bonding, wafer bonding (e.g.,direct bonding interface), through-oxide vias (TOVs), through-siliconvias (TSVs), metal bonding, and/or any other suitable bondinginterfaces.

In one embodiment, the system can include elements implemented in asilicon photonics platform (e.g., implemented by one or more foundriessuch as APSUNY, IME, IMEC, GlobalFoundries, TSMC, etc.), which caninclude silicon, silicon doping, silicon oxides, passive siliconcomponents (e.g., waveguides, filters, etc.), and/or germanium-basedelements (e.g., detectors, filters and/or modulators, such as EAMmodulators, etc.). Additionally or alternatively, the system can includeelements implemented in one or more III-V platforms (e.g., JePPiXconsortium SMART Photonics and/or HHI platforms, Infinera, AIMPhotonics, etc.), which can include materials such as indium compounds,phosphide compounds, gallium compounds, arsenide compounds, and/or anyother suitable III-V semiconductors (e.g., InP substrate with InGaAsPfeatures). In an example of this embodiment, the emitters (e.g., laserarray) are fabricated in the III-V semiconductor platform, themultiplexer is fabricated in either the III-V semiconductor platform orthe silicon photonics platform, and substantially all other photonicelements of the system (e.g., except some or all waveguides associatedwith the emitters) are fabricated in the silicon photonics platform. Insome examples, the elements can be co-integrated with elementsimplemented in an electronics platform (e.g., integrated such asdescribed above regarding packaging technologies). In some suchexamples, one or more electronic elements (e.g., transistors) arefabricated in the photonics platform rather than the electronicsplatform (e.g., thereby enabling and/or facilitating use of high-voltageelements that exceed the voltage limits of the electronics platform).For example, in a system in which elements from a 7 nm electronicsplatform (e.g., with a 0.7 V limit) are coupled with elements from asilicon photonics platform, the silicon photonics platform elements caninclude transistors (e.g., configured to amplify signals received fromthe electronics platform elements) operating with voltages in excess ofthe electronics platform limit.

The system can additionally or alternatively include elementsimplemented in a zero-change silicon photonics platform (e.g., platformtypically used for microelectronics), preferably wherein some or allphotonic and electronic elements of the system are implementedmonolithically (e.g., collocated in the same integrated circuit).Additionally or alternatively, the systems can include elementsimplemented in a co-integrated electronic and photonic platform, such asone that includes front-end-of-line (FEOL) modifications to a standardmicroelectronic fabrication process and/or back-end-of-line (BEOL)modifications for the fabrication of integrated photonic components(e.g., with low capacitance links to the electronics).

The system can additionally or alternatively include elementsimplemented in a hybrid silicon/III-V photonics platform, such aswherein silicon photonics elements and III-V photonics elements (e.g.,optical amplifiers, laser sources, etc.) are implemented monolithically(e.g., collocated in the same integrated circuit). For example, a III-Vsemiconductor substrate (e.g., InP) can support both the siliconphotonics elements and III-V photonics elements.

The system can additionally or alternatively include elementsimplemented in a silicon nitride photonics platform (e.g., JePPiXconsortium TriPLeX platform), such as including waveguides defined bysilicon nitride within a silicon oxide.

The system can additionally or alternatively include elementsimplemented in a silicon-graphene photonics platform, such as whereinone or more photonic elements (e.g., active elements, such as detectors,filters, modulators, etc.) are implemented using graphene, othergraphitic materials, and/or other 2-D materials.

The system can additionally or alternatively include elementsimplemented in a lithium niobate photonics platform, which can includeone or more photonic elements implemented using lithium niobate, such asthin-film lithium niobate.

In a specific example, the system includes elements fabricated such asdescribed in U.S. Pat. No. 10,009,135, issued 26 Jun. 2018 and titled“System and Method for Photonic Processing”, which is herebyincorporated in its entirety by this reference (e.g., as describedregarding fabrication on silicon-on-insulator wafers).

A person of skill in the art will recognize that the elements describedherein using the term “waveguide” can additionally or alternativelyinclude any other suitable optical paths and/or elements associated withoptical paths (including, without limitation, free-space paths and/orpaths including free-space segments).

However, the system can additionally or alternatively be implemented inany other suitable material platform, and can additionally oralternatively include any other suitable materials.

2. Method

A method 200 is preferably implemented using the system 100 describedabove, but can additionally or alternatively be implemented using anyother suitable system(s). The method preferably includes: controllingthe computation module S210; controlling the input module S220; and/orreceiving outputs from the computation module S230 (e.g., as shown inFIG. 9).

In some embodiments, the method 200 includes one or more elements suchas described in U.S. Pat. No. 10,009,135, issued 26 Jun. 2018 and titled“System and Method for Photonic Processing”, which is herebyincorporated in its entirety by this reference (e.g., implementing themethod of U.S. Pat. No. 10,009,135 using the system 100 describedherein). However, the method can additionally or alternatively includeany other suitable elements.

Controlling the computation module S210 preferably includes controllingone or more of the spectral filter banks, more preferably controllingall the spectral filter banks. For example, S210 can include applyingcontrol voltages to one or more optical filters, thereby controlling theoptical filters' interaction with the optical signal (e.g., defining thematrix by which the input vector is multiplied).

Controlling the input module S220 preferably includes controlling one ormore transducers to emit light (e.g., light encoding an input signal,such as the input vector). The emitted light preferably propagatesthrough the computation module, thereby causing the desired calculationto be performed (e.g., multiplying the input vector by the matrix). Inone example, the emitted light interacts with the spectral filter banks,thereby being filtered (e.g., according to the spectral filter weightsassociated with the matrix), and is then sampled by the detectorsgenerating an output signal.

Receiving outputs from the computation module S230 preferably functionsto sample the results of the computation. The outputs are preferablyreceived from the detectors, but can additionally or alternatively bereceived from any other suitable elements. In a first embodiment, dataassociated with the received outputs is stored. For example, an analogelectrical signal (e.g., signal generated at the detector, signalderived from one or more detector signals, such as described below,etc.) can be converted to a digital signal, optionally transformed intoa derived signal (e.g., as described below), and stored in a storageelement of the control module (e.g., RAM). Generating a derived signalcan include, for example, combining (e.g., adding or subtracting)signals from multiple detectors, applying one or more functions (e.g.,nonlinear functions) to the signal(s), and/or any other suitable signaltransformations. The derived signals can be generated in the analogdomain and/or in the digital domain. In a second embodiment, thereceived outputs are used to drive one or more transducers (e.g.,transducers of the same input module, of another input module, etc.).For example, the received outputs can be used to drive transducers suchas described in U.S. Pat. No. 10,009,135, issued 26 Jun. 2018 and titled“System and Method for Photonic Processing”, which is herebyincorporated in its entirety by this reference (e.g., as describedregarding FIG. 2 of U.S. Pat. No. 10,009,135). In a third embodiment,the generated and/or derived signal can be re-modulated (e.g., onto adifferent wavelength) and transmitted into a photonic computing element(e.g., in the same portion of the chip, a different portion of the chip,a different chip, etc.). However, the outputs can additionally oralternatively be received and/or used in any other suitable manner.

The method 200 preferable includes repeating S220 (e.g., changing thesignal encoded by the emitted light during each such repetition). Whilerepeating S220, the method preferably includes substantially maintainingthe same spectral filter bank control. However, the method canalternatively include changing the filter bank control (e.g., encoding anew matrix) between (and/or during) different repetitions of S220. Themethod preferably includes continuing to perform S230 throughout therepetitions of S220 (e.g., receiving the outputs associated with eachrepetition of S220).

Repeating S220 can function to enable rapid computation based on manydifferent input signals. In some examples, S220 is repeated at a rate of20 GS/s (e.g., 20 billion different input signals per second), 10 GS/s,5 GS/s, 2.5 GS/s, 1 GS/s, 0.5-2 GS/s, 2-8 GS/s, 8-32 GS/s, and/or anyother suitable rate. In a specific example, in which the system includes256 channels (e.g., 256 emitters) and each channel encodes a 4-bitsignal (e.g., 4 mantissa bits), a repetition rate of 5 or 10 GS/s canresult in an input signal rate of over 5 or 10 Tbit/s, respectively.

In one example, S210 and/or S220 are performed such as shown in FIG. 7B.Although FIG. 7B depicts specific examples of various elements of theinput module 110 (e.g., transducers such as emitters, filters, and/ormodulators) and computation module 120 (e.g., spectral filter banks,detectors, etc.), a person of skill in the art will recognize that theS210 and/or S220 could additionally or alternatively be performed usinga system with any other suitable examples of these elements (e.g., asdescribed above regarding the system 100).

In embodiments in which the method is limited to performing matrixoperations at B bits of precision for a weight bank, the method canoptionally include enhancing some or all such operations to a higherlevel of precision by using multiple weight banks, such as M·B bits ofprecision for M² weight banks (e.g., wherein M is a positive integer).In one example, in which M=2, wherein x_(Q) represents signal x at Qbits of precision, and x_(Q(i)) represents the ith most significant setof Q bits of a higher-precision representation of x_(Q):

$\begin{matrix}{{\sum{w_{2B}x_{2B}}} = {\sum\left\lbrack {\left( {w_{B} + \frac{w_{B{(2)}}}{2^{B}}} \right)\left( {x_{B} + \frac{x_{B{(2)}}}{2^{B}}} \right)} \right\rbrack}} \\{= {{\sum{w_{B}x_{B}}} + {\frac{1}{2^{B}}\left( {{\sum{w_{B}x_{B{(2)}}}} + {\sum{w_{B{(2)}}x_{B}}}} \right)} + {\frac{1}{2^{2B}}{\sum{w_{B{(2)}}x_{B{(2)}}}}}}}\end{matrix}$

wherein x_(Q) refers to signal x at Q bits of precision, and the sumsare taken over all optical channels (e.g., wavelengths). Each of the M²terms is preferably implemented using a separate weight bank (and/orusing the same weight bank at separate times, such as consecutivetimes), and the detector outputs from the different weight banks (and/ordifferent calculation iterations using the same weight bank) arepreferably summed digitally (e.g., by the control module), but canadditionally or alternatively be summed (and/or otherwise combined) inany other suitable manner.

In alternate embodiments, the method includes performing matrixoperations (e.g., matrix multiplication, such as vector-matrixmultiplication) for matrices of sizes greater than the number ofwavelengths (e.g., the number of transducers) of the system (e.g., asdescribed above regarding alternate embodiments of the computationmodule 120), such as shown by way of example in FIG. 8C. The process ofcomputing such matrix operations (and/or any suitable elements thereof)is preferably performed for each wavelength and/or other optical channel(e.g., L different times for an L-channel system), more preferablywherein the procedure is performed concurrently (or substantiallyconcurrently) for all L optical channels (e.g., by concurrentlyproviding optical inputs associated with each channel and controllingfilter elements associated with each channel). The result can optionallybe translated to a set of control signals (e.g., voltage values) on aseries of wavelength-selective elements (e.g., phase modulator elements)and/or amplitude modulators, and/or can be used in any other suitablemanner (e.g., as described above in more detail, such as regardingS230).

However, the method can additionally or alternatively include any othersuitable elements performed in any suitable manner.

An alternative embodiment preferably implements the some or all of abovemethods in a computer-readable medium storing computer-readableinstructions. The instructions are preferably executed bycomputer-executable components preferably integrated with acommunication routing system. The communication routing system mayinclude a communication system, routing system and a pricing system. Thecomputer-readable medium may be stored on any suitable computer readablemedia such as RAMs, ROMs, flash memory, EEPROMs, optical devices (CD orDVD), hard drives, floppy drives, or any suitable device. Thecomputer-executable component is preferably a processor but theinstructions may alternatively or additionally be executed by anysuitable dedicated hardware device.

Although omitted for conciseness, embodiments of the system and/ormethod can include every combination and permutation of the varioussystem components and the various method processes, wherein one or moreinstances of the method and/or processes described herein can beperformed asynchronously (e.g., sequentially), concurrently (e.g., inparallel), or in any other suitable order by and/or using one or moreinstances of the systems, elements, and/or entities described herein.

The FIGURES illustrate the architecture, functionality and operation ofpossible implementations of systems, methods and computer programproducts according to preferred embodiments, example configurations, andvariations thereof. In this regard, each block in the flowchart or blockdiagrams may represent a module, segment, step, or portion of code,which comprises one or more executable instructions for implementing thespecified logical function(s). It should also be noted that, in somealternative implementations, the functions noted in the block can occurout of the order noted in the FIGURES. For example, two blocks shown insuccession may, in fact, be executed substantially concurrently, or theblocks may sometimes be executed in the reverse order, depending uponthe functionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts, or combinations of special purpose hardware andcomputer instructions.

As a person skilled in the art will recognize from the previous detaileddescription and from the figures and claims, modifications and changescan be made to the preferred embodiments of the invention withoutdeparting from the scope of this invention defined in the followingclaims.

We claim:
 1. A system comprising a spectral filter bank, the spectralfilter bank comprising: a first waveguide comprising a first input endand a first output end; a second waveguide comprising a second input endand a second output end; a first optical filter associated with a firstoptical characteristic, the first optical filter optically coupled tothe first waveguide between the first input end and the first outputend; a second optical filter associated with a second opticalcharacteristic different from the first optical characteristic, thesecond optical filter optically coupled to the first waveguide betweenthe first input end and the first output end; and an optical coupler,optically coupled to the first output end and the second output end, theoptical coupler configured to: receive a first optical signal at thefirst output end; receive a second optical signal at the second outputend; generate an optical interference signal by optically coupling thefirst and second optical signals; and provide the optical interferencesignal at a coupler output.
 2. The system of claim 1, further comprisingan optical splitter comprising a splitter input, a first splitteroutput, and a second splitter output, wherein: the optical splitter isconfigured to: split an optical input signal, received at the splitterinput, into a first optical output signal and a second optical outputsignal; provide the first optical output signal at the first splitteroutput; and provide the second optical output signal at the secondsplitter output; the first splitter output is optically coupled to thefirst input end; and the second splitter output is optically coupled tothe second input end.
 3. The system of claim 2, further comprising: athird waveguide comprising a third input end, a third output end, and athird coupling region between the third input end and the third outputend; a fourth waveguide comprising a fourth input end, a fourth outputend, and a fourth coupling region between the fourth input end and thefourth output end; a third optical filter associated with the firstoptical characteristic, the third optical filter optically coupled tothe third waveguide between the third input end and the third couplingregion; a fourth optical filter associated with the second opticalcharacteristic, the fourth optical filter optically coupled to the thirdwaveguide between the third input end and the third coupling region; anda second optical coupler that optically couples the third couplingregion and the fourth coupling region; wherein the optical splitter isconfigured to: further split the optical input signal into a thirdoptical output signal and a fourth optical output signal; provide thethird optical output signal at a third splitter output of the opticalsplitter, the third splitter output optically coupled to the third inputend; and provide the fourth optical output signal at a fourth splitteroutput of the optical splitter, the fourth splitter output opticallycoupled to the fourth input end.
 4. The system of claim 1, furthercomprising an optical detector optically coupled to the coupler output.5. The system of claim 4, wherein the optical detector comprises aphotodiode.
 6. The system of claim 4, wherein: the optical coupler isfurther configured to: concurrent with generating the opticalinterference signal, generate a second optical interference signal; andprovide the second optical interference signal at a second coupleroutput; and the system further comprises a second optical detectoroptically coupled to the second coupler output.
 7. The system of claim1, wherein: the first optical characteristic is a first opticalwavelength; the second optical characteristic is a second opticalwavelength; the first optical filter comprises a firstwavelength-selective filter; and the second optical filter comprises asecond wavelength-selective filter.
 8. The system of claim 7, wherein:based on a first control signal, the first wavelength-selective filteris configured to phase-shift a first portion of light within the firstwaveguide, the first portion having the first optical wavelength; andbased on a second control signal, the second wavelength-selective filteris configured to phase-shift a second portion of light within the firstwaveguide, the second portion having the second optical wavelength.